1. Field of the Invention
The present invention relates to the coupling of plastic pipes by welding them by electrical resistance heating.
More specifically, the invention relates to the coupling of thermoplastic pipes made of materials such as polypropylene and polyethylene, by inserting therebetween a connecting piece made of the same material and welding the elements together by means of an electrical heating resistance. The connecting piece is used instead of a sealing member.
When a few volts of current are supplied to the electrical resistance element, the plastic material is caused to melt both at the male end of one of the pipes and at the socket end of the other pipe. A uniform weld is thereby obtained. A sleeve comprising two socket ends can obviously be used for pipes which do not have a socket end.
To produce the above-mentioned uniform weld, it is necessary to apply equal amounts of heat to the male end and the socket end.
2. Description of the Prior Art
The conventional connecting pieces for producing a weld of the above-defined type are produced by injecting a plastics material about an electrical resistance element in the form of a bifilar helicoidal winding designed to be internally embedded in the manner of an armature.
Connecting pieces of this type are difficult to manufacture owing to their reduced thickness which is limited by the annular space available between the male end of one pipe and the socket end of another pipe or sleeve. The tubular elements to be coupled generally have a small nominal diameter which may vary between 50 and 150 mm and rarely exceeds 200 mm. Their thickness does not exceed a few millimeters.
Accordingly, it is difficult to satisfactorily embed the resistance element coaxially in the injected plastic material in such a way that the winding turns of the resistance element are equidistant, thereby avoiding any contact capable of producing a short circuit. It is also difficult to cover this resistance element with plastics material in a uniform manner and to avoid any asymmetry or irregularity in its final position within the connecting piece. Consequently, connecting pieces produced in this manner sometimes have the disadvantage of irregularly heating the tubular ends to be connected.
For this reason, attempts were made to eliminate the problem of covering an electrical resistance by injection by externally coiling the resistance element about a temporary support connection and by locating this resistance on the inner surface of the socket end via its temporary support and in finally removing the support by deformation.
It was thereby possible to produce socket ends provided with an electrical heating resistance element which was disposed in a uniform and suitably centered fashion but this did not solve the problem of adapting it to existing tubular elements in place of a sealing member without altering the ends to be connected.
Efforts were also made to produce helicoidal windings consisting of contiguous turns of electrical wire sheathed in plastic designed to be inserted directly between the pipe ends to be connected after welding together the turn sheathings. However, it was not possible to retain the contiguous turns. Even if these turns can be rendered contiguous, they cease to be thus when the sheathed winding is inserted between the ends to be connected.